Nuclear fuel assembly with reinforced flow channel

ABSTRACT

A nuclear fuel assembly wherein the lower portion of the flow channel is reinforced or stiffened in the region of its interface with the lower tie plate for improved control of bypass flow of coolant into the spaces between the fuel assemblies of a nuclear reactor.

Melted States atent [191 Vernier et a1.

1 1 Feb. 6, 1973 1541 NUCLEAR FUEL ASSEMBLY WITH REINFORCED FLOW CHANNEL[75] Inventors: Dominic A. Venicr; Bart A. Smith; James L. Lass, all ofSan Jose, Calif.

[73] Assignee: General Electric Company [22] Filed: Aug. 15, 1969 [21]Appl. No.: 850,486

[52] U.S. Cl ..176/78, 176/76 [51] int. Cl. ..G2lc 3/34 [58] Field ofSearch ..176/76, 78, 87, 73

[56] References Cited UNITED STATES PATENTS 3,104,218 9/1963 Speidel eta1. 176/78 3,137,635 6/1964 Moore et a1. ..176/78 X 3,158,549 11/1964Fowler ..176/78 3,309,280 3/1967 Balog ..176/61 3,317,399 5/1967 Winders176/78 3,350,275 10/1967 Venier et a1. 176/76 X 3,368,946 2/1968.lenssen ..176/78 3,395,077 7/1968 Long Sun Tong et a1. ..176/783,481,021 12/1969 Glandin et al. ..176/78 X 3,344,036 9/1967 Haslam etal. ..176/76 X Primary Examiner-Carl D. Quarforth AssistantExaminer-Gary G. Solyst Attorney-Ivor .1. James, Jr., Samuel E. Turner,Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACTA nuclear fuel assembly wherein the lower portion of the flow channel isreinforced or stiffened in the region of its interface with the lowertie plate for improved control of bypass flow of coolant into the spacesbetween the fuel assemblies of a nuclear reac- 9 Claims, 10 DrawingFigures PATENTEDFEB s :375

SHEET 1 OF 6 Ill mvau'rons DOMINIC A.VEN|ER BART A. SMITH JAMES L. LASSATTORNEY PATENTEDFEB 6l973 3.715274 SHEET 20F 6 PATENTEDFEB 6 ms SHEET30F 6 Fig. 3

PAIENTED FEB 6 i175 SHEET 6 OF 6 I N m T c L V m A .m 1 II S R L I H m F1 T R OE P F 6 6.. NL I 1 N I 0 w T I m R D A 1 EN 1 I BE R 1 O I I u RI P I F 4 2 m 8 6 4 2 Fig. 7

NUCLEAR FUEL ASSEMBLY WITH REINFORCED FLOW CHANNEL BACKGROUND In a knowntype of nuclear reactor, for example as used in the Dresden NuclearPower Station near Chicago, Illinois the reactor core is of theheterogeneous type. That is, the core comprises a plurality of fuelassemblies vertically arranged in an array to form the nuclear reactorcore capable of self-sustained nuclear fission reaction. The core iscontained in a pressure vessel wherein it is submersed in a workingfluid, such as light water, which serves both as a coolant and as aneutron moderator. A plurality of control rods, containing neutronabsorbing material, are selectively insertable among the fuel assembliesto control the reactivity of the core.

Each fuel assembly comprises a tubular flow channel containing an arrayof elongated, cladded fuel elements or rods supported between upper andlower tie plates. The fuel assemblies are supported in the pressurevessel between an upper core grid and a lower core support plate. Thelower tie plate of each fuel assembly is formed with a nose piece whichseats in a fuel support casting which in turn fits through an aperturein the core support plate into a pressurized coolant supply chamber. Thenose piece is formed with openings through which the pressurized coolantflows upward through the fuel assembly flow channels to remove heat fromthe fuel elements. A typical fuel assembly of this type is shown, forexample, by D. A. Venier et al. in US. Pat. No. 3,350,275. In nuclearreactors of recent design in-core nuclear instrumentation, in the formof neutron detectors, are contained in instrumentation receptacleslocated in the gaps between the fuel assemblies.

In a water reactor heat is transferred from the fuel through the fuelrod cladding to the water flowing upward among the fuel rods. At someelevation the flowing water reaches saturation temperature and beyondthis point increasing fractions of the water are in the vapor phase.Normally, the heat transfer coefficient between the fuel rod claddingand the water is substantially constant. However, if the heat-flux isincreased sufficiently, a threshold is reached at which the heattransfer coefficient decreases suddenly by a factor of 5 to 10. This iscaused by a change in the heat transfer mechanism from nucleate boilingto film boiling and it results in a very rapid, undesirable rise in fuelrod cladding temperature. The heat flux at the threshold betweennucleate boiling and film boiling is designated the critical heat flux.

An important consideration in the design of boiling water reactors isthe relationship between the in-channel flow or the coolant flow throughthe fuel assembly flow channels and the bypass flow or the coolant flowthrough the gaps among the fuel assemblies. 0n the one hand it isdesirable to maximize the in-channel flow to thereby maximize the marginto critical heat flux. 0n the other hand it is necessary to provide alimited amount of bypass flow to avoid coolant stagnation and steamvoids and adequately to cool the control rods and the in-coreinstrumentation devices located in the gaps between the fuel assemblyflow channels. Thus for a given total core recirculation flow, thedesirable balance between in-channel and bypass flow maintains anadequate margin to critical heat flux while avoiding excessiveout-of-channel voids.

In prior arrangements control of bypass flow is accomplished by allowingan amount of coolant leakage between the assembly flow channel and thelower tie plate. The flow channel is not fixed to the fuel assembly butis instead a slip fit over the upper and lower tie plate so that itreadily can be removed during refueling and for inspection of the fuelassemblies and rods. The flow channel is formed of relatively thinmaterial to minimize space and to minimize parasitic neutron absorptionand it is found that increases in pressure of the coolant (to increasecoolant flow through the fuel assemblies) causes the flow channel todeflect away from the lower tie plate thus causing an excessive amountof bypass flow with the danger of depriving the fuel assembly ofitsrequired coolant flow.

The prior arrangements for bypass flow have been found inadequate fornuclear reactors of recent design which operate at higher power density,higher steam qualities and lower thermal margins. Thus it is founddesirable to provide more stringent and accurate control of the bypassflow.

SUMMARY It is an object of the invention to provide an improved fuelassembly incorporating a bypass flow control arrangement which maintainsthe bypass flow at a substantially constant percentage of the total coreflow and which does not unduly compromise the ease of removing andplacing the flow channel.

These and other objects are accomplished according to the presentinvention by providing reinforcing or stiffening of the lower end of theflow channel in the region of the interface between the flow channel andthe lower tie plate to enhance control of leakage flow between thechannel and the lower tie plate.

In accordance with a first illustrated embodiment the lower portion ofthe flow channel is formed with circumferential corrugations to increaseits stiffness.

In accordance wit a second illustrated embodiment the lower portion ofthe flow channel is formed with a greater thickness for increasedstrength and stiffness.

In accordance with a third illustrated embodiment the lower portion ofthe flow channel is reinforced by a sleeve surrounding and attached tothe lower portion of the flow channel. The reinforcing sleeve may beplain or corrugated. In an alternate version of the third embodimentreinforcement is provided by a series of spaced narrow sleeves or rings.

In accordance with a fourth illustrated embodiment the lower portion ofthe flow channel is reinforced by hemming, that is, by forming thechannel extra long and by folding the extra length of the channel wallsback along the lower portion of the channel.

BRIEF DESCRIPTION OF THE DRAWING The invention is described morespecifically hereinafter with reference to the accompanying drawingwherein:

FIG. 1 is a schematic illustration of a nuclear reactor steam generator;

FIG. 2 is a partly cutaway perspective view of a fuel assembly;

FIG. 3 is a partly cutaway perspective view of the flow channelillustrating the corrugated reinforced lower portion of the flow channelof the first embodiment;

FIG. 4a is a partly cutaway perspective view of the flow channelillustrating the thickened lower portion of the second embodiment;

FIG 4b is a partly cutaway perspective view of the flow channelillustrating a grooved or corrugated version of the thickened,reinforced portion of the flow channel;

fIG. 5a is a partly cutaway perspective view of the flow channelillustrating the reinforcing sleeve around the lower portion of thechannel in accordance with the third embodiment;

FIG. 5b is a partly cutaway perspective view of the flow channelillustrating a corrugated version of the reinforcing sleeve;

FIG. 5c is a partly cutaway perspective view of the flow channelillustrating a series of spaced, narrow reinforcing sleeves or rings;

FIG. 6 is a partly cutaway perspective view of the flow channelillustrating the hemmed lower portion of the flow channel in accordancewith the fourth embodiment; and

FIG. 7 is a graphical illustration of the performance of the presentinvention compared to the prior art arrangement.

GENERAL DESCRIPTION The invention is described herein in connection witha water cooled and moderated nuclear reactor, an example of which isillustrated in FIG. 1. Such'a reactor system includes a pressure vessel10 containing a nuclear chain reactor core 11 submersed in a coolantsuch as light water. The core 11 is surrounded by an annular shroud 12.The core 11 includes a plurality of replaceable fuel assemblies 13arranged in spaced relation and supported in the vessel 10 between anupper core grid 14 and a lower core support plate 16. Each fuel assemblyincludes a nose piece 17 which engages a support socket in the supportplate 16. The end of the nose piece projects through the support plate16 and is formed with openings for communication with a coolant supplychamber 19. A circulation pump 18 pressurizes the coolant in the supplychamber 19 from which the coolant is forced through the openings in thenose pieces 17 upward through the fuel assemblies. A part of the coolantis thereby converted to steam which passes through a separator-dryerarrangement 20 to a utilization device such as a turbine 21. Condensateformed in a condenser 22 is returned as feedwater to the vessel 10 by apump 23. A plurality of control rods 24 are selectively insertable amongthe fuel assemblies 13 for control of the reactivity of the core. Aplurality of instrumentation receptacles are positioned among the fuelassemblies to contain neutron detectors for monitoring the power levelof the core.

Illustrated in FIG. 2 is a fuel assembly 13 comprising a plurality ofelongated fuel rods 26 supported between a lower tie plate .27 and askeletonized upper tie plate 28. The fuel rods 26 pass through aplurality of fuel rod spacers 29 which provide intermediate support toretain the elongated rods in spaced relation and restrain them fromlateral vibration.

Each of the fuel rods 26 comprises an elongated tube containing thefissionable fuel, in the form of pellets, particles, powder or the like,sealed in the tube by upper and lower end plugs 30 and 31. Lower endplugs 31 are formed with a taper for registration and support in supportcavities 32 which are formed in the lower tie plate 27. Upper end plugs30 are formed with extensions 33 which register with support cavities 34in the upper tie plate 28.

Several of the support cavities 32 (for example, selected ones of theedge or peripheral cavities) in the lower tie plate 27 are formed withthreads to receive fuel rods having threaded lower end plugs 31. Theextensions 33 ofthe upper end plugs 30 ofthese same fuel rods areelongated to pass through the cavities in upper tie plate 28 and areformed with threads to receive internally threaded retaining nuts 35. Inthis manner the upper and lower tie plates and the fuel rods are formedinto a unitary structure.

The fuel assembly 13 further includes a thin-walled tubular flow channel36, of substantially square cross section, adapted to provide a slidingfit over the lower and upper tie plates 27 and 28 and the spacers 29 sothat it readily may be mounted and removed. The channel 36 has a tab 37welded to the top end which provides for fastening the channel to thefuel bundle with a bolt 38.

The lower tie plate 27 is formed with a nose piece 17 adapted, asmentioned hereinbefore, to support the fuel assembly in a socket in thesupport plate 16 (FIG. 1). The end of the nose piece is formed withopenings 39 to receive the pressurized coolant so that it flows upwardamong the fuel rods.

To avoid stagnation of the coolant in the spaces 25 (FIG. 1) among thefuel assemblies, a portion (in the order of 56 percent) of the coolantflow into each fuel assembly is allowed to leak into the adjacent spaces25 from between the lower tie plate 27 and the channel 36 of the fuelassembly as indicated by the arrow legended LF in FIG. 2 to providebypass flow among the channels. As discussed hereinbefore, the priorarrangements have not provided adequate regulation of this bypass flow.In accordance with the present invention control of the leakage flow LPis enhanced by strengthening or stiffening the lower portion 40 oftheflow channel 36 in the region of the interface between the flow channeland the lower tie plate 27. The thus reinforced flow channel moresuccessfully resists deflection away from the lower tie plate withincreased pressure differentials and irradiation life whereby theleakage flow area and consequently the percentage leakage flow remainsmore nearly constant.

If the reinforced or stiffened flow channel is found too restrictive ofbypass flow, additional bypass flow passages, 41 through the lower tieplate 27 and/or 42 through the walls of flow channel 36, may beprovided.

FIRST EMBODIMENT A first embodiment of the invention is illustrated inFIG. 3. In this embodiment the lower, interface portion 40 of the flowchannel 36 is reinforced and stiffened by forming this portion withcircumferential corrugations 43. The corrugations provide greatlyincreased lateral stiffness to deflection of this portion away from thelower tie plate 27. Additionally, the convolutions of the corrugations,being perpendicular to the leakage flow, provide increased flowresistance in the leakage flow path.

SECOND EMBODIMENT A second embodiment of the invention is illustrated inFIGS. 4a and 4b. In this embodiment the lower portion 40 only of theflow channel 36 is formed with a greater thickness 1, sufficient toprovide the desired strength and stiffness in this interface portionwhile the remainder of the flow channel is formed of its normalthickness t As illustrated in FIG. 4b, the outer surface of the thickerportion may be formed with circumferential grooves or corrugations 44for increased stiffness-to-quantity of material ratio.

THIRD EMBODIMENT A third embodiment of the invention is illustrated inFIGS. 5a, 5b and So. In this embodiment the lower, interface portion 40of the flow channel 36 is reinforced and stiffened by a surroundingsleeve. As illustrated in FIG. 5a, a sleeve 46 surrounds the lowerportion 40 of the flow channel 36 and is attached thereto as by weldingat 47. The sleeve 46 may be stress relieved in wellknown manner beforeattachment to channel 36 to decrease creep and relaxation withirradiation and aging.

The reinforcing sleeve may be corrugated for increasedstiffness-to-quantity of material ratio as illustrated by a corrugatedsleeve 46 in FIG. 5b.

A greater strength-to-quantity of material ratio may also be achieved asillustrated in FIG. 5c wherein the interface portion 40 of the flowchannel 36 is reinforced by a series of spaced narrow sleeves or rings46".

FOURTH EMBODIMENT A fourth embodiment of the invention is shown in FIG.6. In this embodiment the interface portion 40 of the flow channel 36 isreinforced and stiffened by hemming. This may be accomplished byoriginally forming the flow channel 36 extra long. The corners of theextra length portion are cut away leaving a reinforcing tab 48 extendingfrom each side of the flow channel. Each of the reinforcing tabs 48 isthen folded outwardly and back against the outer walls of the interfaceportion 40 ofthe flow channel.

While with some materials it is possible to extend the hem around thecorners of the flow channel, this is usually impractical due to thedifficulty of forming the preferred low neutron absorbing zirconiummaterial. Furthermore, it is found that the desired leakage flow controlcan be achieved without extension of the hem around the corners.

The effectiveness of the present invention is graphically illustrated inFIG. 7 which compares the performance of the flow control arrangement ofthe invention with the prior art arrangement which relied only upon apredetermined fit between the flow channel 36 and the lower tie plate27. The solid curves show the performance of new fuel assemblies whilethe dashed curves show end-of-life performance, that is, after theassembly has been operated in a reactor core on the order of severalyears. For the prior art arrangement the leakage flow increases by morethan one-third with life while for the arrangement of the presentinvention the leakage flow change with life is less than one percent.Changes in leakage flow with changes in coolant flow are significantlyless for the arrangement of the present invention. A further outstandingadvantage of the invention is that it does not compromise or complicatethe ready removal and replacement of the flow channel 36.

While illustrative embodiments of the invention have been describedherein, modifications and adaptations thereof may be made by thoseskilled in the art without departure from the spirit and scope of theinvention as defined by the following claims.

What is claimed is:

l. A fuel assembly comprising a plurality of fuel rods positioned in aspaced array by upper and lower tie plates, said lower tie plate havingmeans to receive therethrough a flow of coolant, an open-ended tubularflow channel surrounding said array for conducting said coolant upwardpast said fuel rods, a portion of the lower end of said channel closelysurrounding a peripheral surface of said lower tie plate to limitleakage of said coolant therebetween, said channel being attached tosaid assembly solely at the top of said assembly with readily releasableattachment means and said channel being freed from and detached fromsaid lower tie plate so that said channel readily is removable from saidassembly, means separate from said channel connecting said upper andlower tie plates together and maintaining said fuel rods in said spacedarray independent of said flow channel, said flow channel being formedwith a reinforced portion in the region of interface with said lower tieplate for added resistance to deflection of said portion of said flowchannel away from said lower tie plate.

2. The fuel assembly of claim 1 wherein said lower tie plate is formedwith at least one bypass coolant flow passage directly from the interiorof said lower tie plate to the exterior of said fuel assembly.

3. The fuel assembly of claim 1 wherein said reinforced portion of saidflow channel is formed with circumferential corrugations.

4. The fuel assembly of claim 1 wherein the walls of said reinforcedportion of said flow channel are formed of thicker material than theremainder of the walls of said flow channel.

5. The fuel assembly of claim 4 wherein the outer surface of the thickerwall portion of said flow channel is circumferentially grooved.

6. The fuel assembly of claim 1 wherein said reinforced portion of saidflow channel is surrounded by a reinforcing sleeve.

7. The fuel assembly of claim 6 wherein said sleeve is circumferentiallycorrugated.

8. The fuel assembly of claim 1 wherein said reinforced portion of saidflow channel is surrounded by each of a plurality of spaced sleevesextending circumferentially around said portion 9. The fuel assembly ofclaim 1 wherein the end of said flow channel adjacent said one of saidtie plates is hemmed to form said reinforced portion of said flowchannel.

4 I i t t

1. A fuel assembly comprising a plurality of fuel rods positioned in aspaced array by upper and lower tie plates, said lower tie plate havingmeans to receive therethrough a flow of coolant, an open-ended tubularflow channel surrounding said array for conducting said coolant upwardpast said fuel rods, a portion of the lower end of said channel closelysurrounding a peripheral surface of said lower tie plate to limitleakage of said coolant therebetween, said channel being attached tosaid assembly solely at the top of said assembly with readily releasableattachment means and said channel being freed from and detached fromsaid lower tie plate so that said channel readily is removable from saidassembly, means separate from said channel connecting said upper andlower tie plates together and maintaining said fuel rods in said spacedarray independent of said flow channel, said flow channel being formedwith a reinforced portion in the region of interface with said lower tieplate for added resistance to deflection of said portion of said flowchannel away from said lower tie plate.
 2. The fuel assembly of claim 1wherein said lower tie plate is formed with at least one bypass coolantflow passage directly from the interior of said lower tie plate to theexterior of said fuel assembly.
 3. The fuel assembly of claim 1 whereinsaid reinforced portion of said flow channel is formed withcircumferential corrugations.
 4. The fuel assembly of claim 1 whereinthe walls of said reinforced portion of said flow channel are formed ofthicker material than the remainder of the walls of said flow channel.5. The fuel assembly of claim 4 wherein the outer surface of the thickerwall portion of said flow channel is circumferentially grooved.
 6. Thefuel assembly of claim 1 wherein said reinforced portion of said flowchannel is surrounded by a reinforcing sleeve.
 7. The fuel assembly ofclaim 6 wherein said sleeve is circumferentially corrugated.
 8. The fuelassembly of claim 1 wherein said reinforced portion of said flow channelis surrounded by each of a plurality of spaced sleeves extendingcircumferentially around said portion